Use user-trash icon for Trash action

Summary:
BUG: 391078
FIXED-IN: 19.04.0

Test Plan:
Before:
{F6557553}

After (WITHOUT QT_SCALE_FACTOR=2 gwenview)
{F6557554}

(WITH QT_SCALE_FACTOR=2 gwenview)
{F6557555}

Reviewers: ngraham

Reviewed By: ngraham

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D18391